Color television tube with shadow mask assembly provided with shield for reducing x-ray radiation and the effect of stray magnetic fields



United States Patent Inventor Richard J. Lindeman Elmwood Park, Ill. Appl. No. 817,243 Filed Apr. 4, 1969 Continuation of Ser. No. 570,888, Aug. 8, 1966, now abandoned. Patented Dec. 22, 1970 Assignee Motorola, Inc.

Franklin Park, 111. a corporation of Illinois COLOR TELEVISION TUBE WITH SHADOW MASK ASSEMBLY PROVIDED WITI-l SHIELD FOR REDUCING X-RAY RADIATION AND THE EFFECT OF STRAY MAGNETIC FIELDS 6 Claims, 3 Drawing Figs.

US. Cl. 313/85, 313/313 Int. Cl H0lj 1/52, H01 j 29/46 Field ol'Search 3 13/85(S),

[56] References Cited UNITED STATES PATENTS 2,523,406 9/1950 Wilder 3l5/8X 2,630,543 3/1953 Cage 3l5/8X 2,806,165 9/1957 Law 313/82 3,376,451 4/1968 Mears 315/8X 2,890,362 6/1959 Francken 313/85 3,028,521 4/1962 Szegho 313/85 Primary Examiner-James W. Lawrence Assistant Examiner-V. Lafranchi AnomeyMueller, Aichele & Rauner ABSTRACT: Shield means are provided inside a color television receiver cathode ray tube for preventing overscanned electrons from falling on the viewing screen, for reducing stray magnetic fields affecting the alignment of the electron beams on their proper phosphors and for limiting the escaping from the tube of X-rays produced from the impingement of the electron beams on the shadow mask.

PATENTED 050221970 FIGI ATTORNEYS COLOR TELEVISION TUBE WITH SHADOW MASK ASSEMBLY PROVIDED WITH SHIELD FOR REDUCING X-RAY RADIATION AND THE EFFECT OF STRAY MAGNETIC FIELDS This application is a continuation of my application Ser. No. 570,888, filed Aug. 6, 1966 This invention pertains generally to a cathode ray tube for a color television receiver and more particularly to a shield structure for such a tube.

In a typical color television receiver, the viewing screen is composed of a plurality of triads of phosphor dots, and the beams from three electron guns are scanned across the screen through holes in a shadow mask assembly. The holes are aligned with thedots so that the beams each impinge on a respective dot of the triad so that it produces its characteristic color. Alignment of the electron guns with the holes in the shadow mask and with the respective phosphor dots is critical. For instance, stray magnetic fields independent of the magnetic force which deflects the beams during the scanning of the screen can cause the beams to become shifted thereby causing them to strike the wrong phosphor dot resulting in a poorly or improperly colored picture presentation. Shields have been developed to reduce the effect of these stray magnetic fields, however, they generally increase the size of the tube assembly, place undue stress thereon, and are relatively ineffective.

Should the beams of the electron guns overscan the screen, the electrons can strike the tube walls and be deflected to the viewing screen resulting in extraneous lighting of the edges of the picture presentation. Shields have been developed which fit around the outside periphery of the shadow mask perpendicular and close to the glass envelope to prevent stray electrons from illuminating the screen. These shields, however, are inefficient in that they perform only this one shielding function. Also, in many instances such shields are not entirely effective because gaps must be left between the shield and the glass walls of the tube to prevent glass damage and the gaps permit electrons to escape the shield and strike the tube walls.

Bombardment of the shadow mask assembly by electrons causes X-rays to be emitted from the back side of the shadow mask screen and frame. It is necessary to limit the amount of X-rays pennitted to escape into the room. In the past, this X- ray radiation has been reduced by including some lead in the glass of the tube and using an external shield. Using such an X- ray shield is undesirable, however, because it increases the size and costs of the tube assembly.

It is an object of this invention to provide an improved shield for a cathode ray tube.

It is another object of this invention to provide a shield for a cathode ray tube used in a color television receiver that is efficient, does not increase the size of the tube and is relatively inexpensive to manufacture.

It is a further object of this invention to provide a single shield for a cathode ray tube used in a color television receiver that reduces X-ray radiation from the shadow mask assembly, eliminates illumination of the viewing screen by overscanned electrons and reduces the effect of stray magnetic fields to cause shiftingof the electron beams.

In practicing this invention, a hollow ferrometallic shield is mounted within the envelope of a cathode ray tube for use in a color television receiver. The shield extends rearwardly from the shadow mask frame. The shield has a first open end which has a formed portion that is fixed to a beveled step portion of the shadow mask frame to form a pocket within the shield. The shield and frame form an enclosure surrounding the paths of the electron beams as they are directed toward the viewing screen of the tube. The shield extends along the longitudinal axis of the tube envelope to a position adjacent the effective magnetic field of the deflection yoke with the second end of the shield, which has a lip portion defining an opening therein, opening to the beam source of electrons. The walls of the shield are contoured to the walls of the tube envelope and are spaced in close proximity thereto for substantially the longitudinal dimension of the shield. The shield surrounds the paths of the electron beams when the beams pass beyond the influence of the magnetic field of the yoke, thereby reducing the effect of stray magnetic fields on the beams without interfering with the magnetic field of the yoke. Should the beams be overscanned, the pocket within the shield formed by the shield and shadow mask frame intercept the unusable electrons. IF the electrons are extremely overscanned so that they do not enter the open end of the shield, they will strike the lip portion surrounding the opening into the shield and be reflected back to the beam source. Therefore, the pocket and lip portions prevent the unusable electrons from striking the viewing screen and causing an undesirable glow about its periphery. Because the shield is fixed to the shadow mask frame and extends rearwardly therefrom, it acts to reduce the X-ray radiation from the frame through the tube envelope and to the atmosphere.

In the drawing:

FIG. 1 is a side elevation view of the cathode ray tube of this invention;

FIG. 2 is a cross-sectional view along the lines 2-2 of FIG. 1 and illustrates the shield in accordance with this invention; and

FIG. 3 is a perspective view of the cathode ray tube with the funnel removed to reveal the shield.

In FIG. 1 the cathode ray tube 10, for use with a color television receiver, includes a funnel portion 12 and a front panel or viewing screen 14 joined to form a sealed envelope.

In FIG. 2 the viewing screen 14 is composed of a plurality of triads of phosphor dots 16. Electron beams travel from three guns 17 that are connected to the socket 21 of the tube 10 and are located in the neck 11 of the funnel portion 12. During scanning of the screen 14, these beams travel through holes 22 in the screen 25 of the shadow mask assembly 27 that is secured to the sides of front panel 14 by clips 24. The holes 22 in the screen 25 of the shadow mask assembly are aligned with the triads of dots so that each of the the beams from the electron guns impinge a respective dot of the triad to produce its characteristic color. Accuracy of the beam landing on the screen 14 is critical because shifting of the beam path from that intended by the deflection components can result in the beam missing its correct dots and producing the wrong color in various areas of the screen.

The tube 10 is provided with an internal shield 30 formed from a thin metallic sheet such as annealed cold rolled steel CRS having a thickness, for example, of .003 to .006 of an inch. The shield is made from CRS so that it can be blackened. This is important, because, it is desirable that the shield be able to absorb heat from the shadow mask assembly so that such heat can be dissipated through the tube walls to the atmosphere thereby minimizing shadow mask expansion shift.

As seen in FIG. 3, the shield 30 is hollow and has a first open end joined to the frame of mask assembly 27. This end has a formed portion 33 that is adapted to be fixed, for example by welding, to the beveled stepped portion, or back side 35, of the shadow mask frame to form a pocket 37 within the shield. The shield 30 extends rearwardly along the longitudinal axis of the tube 10 and surrounds the paths of the electron beams. The walls 36 of the shield 30 are contoured to the walls 39 of the tube 10 and are spaced in a close proximity thereto, for example 0.25 inch, for substantially the longitudinal length of the shield.

With the shield 30 mounted to the frame 40 of the shadow mask 27, the opening 38 in the second end of the shield 30 is spaced some distance from the electron guns 17 in the neck 11 of the tube 10 and adjacent the extent of the effective magnetic field of the deflection yoke 41. Spacing the metallic shield 30 from the deflection yoke, insures that the metal of the shield 30 will not interfere with the magnetic field of the yoke. Therefore, it eliminates any need for an increase in power in the deflection yoke. If the shield 30 extended from the electron guns to the shadow mask assembly 27, the metal,

of the shield would interfere with the magnetic field of the yoke. requiring a relatively high powered yoke to deflect the beams during scanning. In addition, some beam shifting could result which would be unacceptable considering the high degree of accuracy of beam landing that is required.

In operation, the electron beams from the guns are deflected by the deflection yoke at the apparent deflection center 43 of the yoke into the shield 30 through the open neck portion 38 of the shield, and the useful electrons are scanned over the viewing screen 14in known manner.

The beam path 20 illustrates an electron beam scanned to a maximum angle and striking the screen 14 at the far edge. Beam path 19 is slightly overscanned but does not strike the concave entry portion of the shield walls but rather passes into the shield near the mask where it strikes the shield at the wall of the formed portion 33 so that secondaries therefrom are collected in the pocket 37 and are prevented from falling on the viewing screen 14 and causing a glow. Beam path 18 represents overscanning more than beam 20 to the point where it misses the opening 38 in the shield. In this instance, however, the lip portion 47 that defines the opening 38 intercepts the electron beam and deflects the electrons back towards the electron guns 17. Because the walls of the shield 30 are contoured to the walls of the tube and in close proximity thereto, an electron, that for some unusual reason was greatly overscanned, could not pass between the tube and shield walls and onto the screen. Instead, the greatly overscanned electrons are trapped in the neck of the tube as shown by beam 45. In addition, because the shield is at the same potential as the viewing screen 14 any electrons that hit it would be quickly attracted to it to provide further protection against the overscanned electrons falling to the viewing screen.

Because the walls of the shield extend generally along the longitudinal axes of the tube 10, the shield 30 surrounds the useful electrons during a major part of their travel after they emerge from the magnetic field of the deflection yoke and pass to the viewing screen 14. By shielding the beams in this manner, with the metallic shield 30, the electron beams are protected from stray magnetic fields including the earths magnetic field which tend to cause beam shifting with resultant loss in picture brightness and color definition. Maximum shielding effect is achieved by limiting opening 38 of the shield to a size which just allows useful electron beams to pass through.

By fixing the formed portion 33 of the shield to the back side of the support frame 40 of shadow mask assembly 27 such that the shield extends rearwardly from the shadow mask assembly, the shield 30 serves as an excellent shield for reducing the amount of X-ray radiation caused by electrons striking the metal of the shield, from the back of the shadow mask assembly 27 to the atmosphere.

The described shield further covers most of the glass walls of the color tube envelope so that a beam cannot strike the walls and cause the release of undesired gases within the tube. It may also be possible in some applications to use the shield 30 as the anode capacitance in place of the usual Aquadag coating within the funnel of the tube.

What has been described, therefore, is a shield for a cathode ray tube for use in a color television receiver that is relatively inexpensive, that reduces X-ray radiation from the shadow mask assembly, that reduces the effect of stray magnetic fields on the deflection of the electron beams and that eliminates illumination of the viewing screen by overscanned electrons striking the tube walls and falling onto the viewing screen.

lclaim:

1. In a cathode ray tube for a color television receiver having a glass funnel portion and a viewing screen portion cooperating to form a sealed envelope and including a beam source for electrons in the funnel portion of the tube to be deflected by a yoke mounted to the tube so that the electron beam scans the viewing screen portion, the combination including, a shadow mask assembly including a shadow mask screen, a hollow ferrometallic shield positioned within the envelope and having first and second open ends, said first open end of said shield being integral with and sealed about the periphery thereof to said shadow mask assembly with the shield extending rearwardly therefrom, said assembly and said shield forming an enclosure surrounding the path of the electron bearn as said beam is directed toward the viewing screen, said shield extending along the longitudinal axis of the envelope to a position adjacent the effective magnetic field of said yoke with said second end opening to the beam source of the electrons for receiving the beam therethrough, said shield surrounding the path of the electron beam with said electrons passing beyond the influence of the magnetic field of the yoke and acting to reduce the effect of stray magnetic fields on said beam without interfering with the magnetic field of the yoke, and said shield being sealed to said shadow mask assembly forming means therebetween to prevent electrons from the beam from passing between said shield and said shadow mask assembly with the beam being overscanned thereby preventing the overscanned electrons from falling on the viewing screen portion and generating an undesirable glow thereon.

2. The cathode ray tube of claim 1 wherein said shadow mask assembly includes a frame having a beveled stepped portion and said first end of said shield has a formed portion that is fitted to said stepped portion and fixed thereto, said formed portion and said stepped portion cooperating to form a pocket for receiving undesired electrons caused by overscanning and to prevent the same from falling on the viewing screen thereby causing an undesirable glow on the screen, and said rearwardly extending shield reducing X-ray radiation caused by electrons striking said pocket or the inside surface of the screen.

3. In a cathode ray tube for a color television receiver having a glass funnel portion and a viewing screen portion cooperating to form a sealed envelope and including a beam source for electrons in the funnel portion of the tube to be deflected by a yoke mounted to the tube so that the electron beam scans the viewing screen portion, the combination including, a shadow mask assembly including a frame for mounting the assembly in the envelope and a shadow mask screen, a hollow ferrometallic shield having first and second open ends, said first end of said shield being supported within the envelope with the shield extending rearwardly from said shadow mask frame, said frame having a first portion and said shield having a second portion that cooperates to form a pocket for receiving undesired electrons caused by overscanning and to prevent the same from falling on the viewing screen, said frame and said shield also forming an enclosure surrounding the path of the electron beam as said beam is directed toward the viewing screen, said shield extending along the longitudinal axis of the envelope to a position adjacent the effective magnetic field of said yoke with said second end opening to the beam source of the electrons for receiving the beam therethrough, said shield surrounding the path of the electron beam with said electrons passing beyond the influence of the magnetic field of the yoke and acting to reduce the effect of stray magnetic fields on said beam without interfering with the magnetic field of the yoke.

4. The cathode ray tube of claim 3 wherein the walls of said shield are contoured to the walls of the tube envelope and are spaced in a close proximity theretofor substantially the longitudinal dimension of said shield so that the useful scanned electrons pass through said second open end of said shield and onto the viewing screen without striking the walls of said shield, and overscanned electrons are prohibited by said close spacing of said shield walls to the envelope walls from passing between the same and falling on the viewing screen and causing a glow around the edge thereof.

5. The cathode ray tube of claim 3 wherein said second open end of said shield has a lip portion about the periphery of the opening therein, said lip portion acting to intercept overscanned electrons that do not enter said shield through said second open end and reflect the same back towards the beam source so that the overscanned electrons are prevented from reaching the viewing screen and causing an undesirable glow thereon.

6. In a cathode ray tube for a color television receiver having a funnel portion and a viewing screen portion cooperating to form a sealed envelope and including a plurality of beam sources for electrons in the funnel portion of the tube to be deflected by a yoke mounted to the tube so that the electron beams scan the viewing screen portion, the combination including, a shadow mask assembly including a frame for mounting the assembly in the envelope and a shadow mask screen, said frame having a beveled stepped portion, a hollow ferrometallic shield having first and second open ends, said first end having a formed portion, said shield being mounted to said frame and extending rearwardly therefrom by fixing said formed portion to said stepped portion, said frame and said shield forming an enclosure surrounding the paths of the electron beams as each said beam is directed toward the viewing screen, said stepped portion and said formed portion cooperating to form apocket, said shield being contoured to the walls of the envelope and extending along the longitudinal axis of the envelope in a close proximity to the walls thereof to a position adjacent the effective magnetic field of said yoke, said second end having a lip portion defining the opening therein and opening to the beam sources of the electrons for receiving the beams therethrough, said shield surrounding the paths of the electron beams with said electrons passing beyond the influence of the magnetic field of the yoke and acting to reduce the effect of stray magnetic fields on said beams without interfering with the magnetic field of the yoke, with said beams being overscanned said pocket intercepting a portion of the unuseful electrons and said lip portion about said second end intercepting another portion of the unuseful electrons and reflecting them back towards the beam source so that said pocket and said lip portion prevent the unuseful electrons from striking the viewing screen and causing an undesirable glow thereon, and said rearwardly extending shield being fixed to said shadow mask frame thereby reducing X-ray radiation from the frame and said. shadow mask screen through the tube envelope. 

